1. Field of the Invention
This invention relates to a control unit in which a plastic case houses power components such as the transistors of an inverter and a smoothing capacitor that are used in an AC motor control apparatus or the like.
2. Description of the Related Art
A control apparatus for an AC motor which drives the spindle of a machine tool uses the circuitry shown in FIG. 4. In order to house this control apparatus in a case 10, it is necessary to have terminals for the illustrated three-phase power supply 14, 15, 16, terminals 18, 19, 20 for feeding current to an AC motor, a ground terminal 26, and terminals 28, 30, T.sub.2 -2 for a thermostat 32. Principal components such as an electromagnetic contactor 34 rectifying diode 36 smoothing capacitor 38 regenerative transistor 40 transistor inverter 42 and filter 44 are disposed inside the case 10, and a number of control elements such as resistors, capacitors, fuses and diodes are disposed in the case 10 and connected to control lines 46.
Control components of this kind are fixed to a mounting board by tightening screws, and conductive portions are connected by soldering, after which the components are accommodated within the case 10, which is made of metal. In this case, it is necessary to mount a number of components which differ in size, and assembly is carried out manually by a skilled worker. A problem that results is poor operating efficiency. Furthermore, since the case is made of metal, the case is heavy and high in cost. Since the sizes of the circuit components differ depending upon the current capacity of the motor, cases of different size must be fabricated. The result is poor productivity.